The energy in flowing fluids such as from wind, flowing rivers and from tides is a source of energy for generating, for example, electricity without producing polluting emissions such as carbon dioxide. At present there is a need for devices that can generate electrical energy from wind or other fluid flows efficiently and economically. Even the extant commercial and industrial devices have an inordinate payback period that makes them very difficult to justify on a purely economic and commercial basis.
There are two major types of wind turbines:                the horizontal axis wind turbine (HAWT) where the wind blows along the axis of rotation, through the ‘disk’ formed by the rotating blades; and        the vertical axis wind turbine (VAWT) where the wind is perpendicular to the axis of rotation.        
The HAWT is a propeller design, with long blades attached at one end to a rotating hub. Once rotating, the apparent wind direction affecting each blade varies along the length of the blade, which is twisted in order to achieve optimum performance. It now seems accepted that only the outer one third of the length of the blade generates appreciable power. Larger sizes of HAWTS create major engineering and installation problems and they are noisy.
The VAWT is exemplified by the Darrieus design first described by Georges Jean Marie Darrieus in U.S. Pat. No. 1,835,018. The turbine has an axle that rotates about an axis that extends perpendicularly to the wind direction. The turbine has a plurality of blades, which are mechanically connected to the axle. The blades have a streamline section, which extends along an axis that is a tangent to a circle concentric with the axle. Wind blowing across the turbine, generates a transverse thrust and thus rotation of the axle.
Recent designs of VAWTs by Turby B. V of Lochem, The Netherlands and XC02/Quiet Revolution (UK Patent Application No. GB-A-2404227) have three blades which rotate about a longitudinal axis in which the upper end and the lower end of the blades are offset from each other horizontally so that each blade has a helix-like form. Other designs for vertical axis wind turbines are disclosed in JP 2008-025518, WO 2008/039727, WO 02/095221, DE 24 44 803, DE 24 51 751, US 2003/0209911 and WO 2008/095389. Other wind turbine designs are disclosed in U.S. Pat. No. 5,332,384 and US 2003/0133782.